1. Field of the Invention
The present invention relates generally to an energy absorption system such as a dynamometer, which incorporates an internally liquid-cooled disc and an optional torque measuring apparatus.
2. Background of the Invention
Disc-braking systems have been used for many years to brake automobiles, aircraft, trucks, and other vehicles. Such braking systems are also used as dynamometers to absorb kinetic energy associated with systems that test power output from power plants, engines and the like. Disc brake systems are chosen over other braking means, such as drum brakes, for various reasons including braking effectiveness (anti-fading), cost, and serviceability.
Generally a disc brake system includes a rotating disc upon which a braking or retarding force is applied. A rotating shaft is connected to the disc, stationary brake pads are forced against the disc to effectuate the braking action, and calipers hold the brake pads in place. In operation, the calipers are normally hydraulically controlled; that is, hydraulic pressure forces the brake pads against the rotating disc. The kinetic energy of the rotating disc is transformed into heat via friction as the disc decelerates between the brake pads. Under normal conditions such as decelerating an automobile operating at a normal highway speed, the heat generated by the energy absorption process is dissipated from the disc and the brake pads to the surrounding air. However, during longer and harder periods of braking, such as braking a car or truck while traversing a long downhill slope found in mountainous terrain, the disc brake system may no longer be cooled at an adequate rate; thereby adversely affecting their braking capability. When such operating conditions are encountered, the brake pads and disc become excessively hot and rapidly become destroyed.
The brake shoe burn-out becomes especially apparent in conditions where there is a high energy transfer rate, such as the kinetic energy transfer associated with rapid braking of a racing car traveling at high speed. In such situations, the use of air to cool the disc brake system is generally insufficient to prevent excessive brake pad wear. Other more effective heat transfer methods have been utilized. One such method involves spraying a liquid, such as water, directly on the rotating disc as it is braking, thus allowing the heat to be transferred to the liquid. This method, though increasing the heat transfer rate, creates a braking hazard because the coefficient of friction between the disc and the pads will vary dramatically as a function of disc/pad temperature and the amount of liquid between the disc and the pad. Thus, an externally liquid-cooled disc brake system, though extending the disc and brake pad lives, can create unreasonable risks and cannot be effectively controlled.
Dynamometers are devices for absorbing and measuring energy output of power plants, engines, or other mechanical energy producing devices (MEPD). By definition, energy per unit of time represents the power generated by the device. Dynamometers are typically used in horsepower output measurement of engines used in racing cars, speed boats, motorcycles, and other high performance machines. Horsepower is simply a measure of power which by definition is equal to 550 foot pounds per second or 745.7 watts.
In operation, the MEPD can be directly connected via its crank shaft, or indirectly coupled, via its power transfer means (e.g. the associated vehicle's drive wheel), to the dynamometer. A retarding torque is applied to the MEPD by the dynamometer. The MEPD's output torque can thus be measured at a given angular speed allowing a horsepower rating to be calculated. The dynamometer must be capable of applying sufficient reactive torque while effectively dissipating the absorbed energy through heat transfer so as to maintain the dynamometer within a safe operating temperature.
Various methods of power absorption have been utilized in dynamometers. A dynamometer employing a disc brake system using air cooled pads and discs has restricted power absorption capability due to heat transfer limits of the air contacting the pads and discs. Such dynamometers are therefore restricted to testing MEPD's of low horsepower capability, with such testing generally limited to short periods of time.
Liquid dynamometers are dynamometers applying reactive torque by means of an impeller in a bath of liquid. Such dynamometers have been used with some success. Here the energy absorption is achieved by heating the liquid through turbulence and either subsequently cooling the liquid in a closed loop system, or using new liquid in an open loop system. Such dynamometers require large amounts of liquid and therefore are not readily portable. Furthermore, because the liquid dynamometer's resistive torque is generally varied by changing the water pressure within the device, it is not easy to change the opposing torque so as to allow testing of different horsepower MEPD's. Finally, a liquid dynamometer is generally expensive to fabricate.
Externally liquid-cooled disc brakes which spray liquid onto the disc to dissipate heat are not particularly suitable for dynamometers since such cooling is generally not uniform, therefore making accurate torque measurement difficult.
Some internally liquid-cooled disc brake systems have been developed to overcome these difficulties. In operation, a liquid is injected into the disc having an internal cavity. The heat generated by the brake is transferred to the liquid, and the liquid, now at a higher temperature, is forced out of the disc. In some instances the amount of energy absorbed by the braking system is large enough to vaporize the liquid, resulting in exiting hot gas (typically steam). The latent heat of vaporization results iun substantially high energy absorption, thereby providing a dynamometer with a higher energy absorption capacity at a given operating temperature than if vaporization did not occur.
Various designs for internal liquid cooling are shown in prior art devices. Eames, U.S. Pat. No. 2,982,377, liquid passing through condutis in the drum as a means of transferring the heat from the friction element. The cooling system is closed loop with the liquid cycled through vehicle's radiator for heat transfer. A pump is used to propel the liquid through the system. Unlike the present invention, the liquid cooled brake of Eames is utilized in a drum rather than a disc and does not use or suggest flow tubes to prevent vapor lock.
Muller, et al., U.S. Pat. No. 2,997,312 discloses an internally liquid cooled disc brake system for use in an automobile. This system, like Eames, is a closed system in the preferred embodiment. Muller et al. uses a forced cooling liquid as a means of heat transfer from the braking disc. The liquid is forced via conduits through the braking discs, which have hallowed passages, or cooling jackets. The cooling liquid is returned to the automobile's radiator for heat transfer. Muller, et al. does not use or suggest cooling cells or flow tubes as disclosed by the present invention in order to maximize cooling capacity and minimize risk of vapor lock.
Dunlop et al., British Patent No. 653,565, employs a sinuous passage in a cast disc to route cooling water through the braking disc to remove the frictional heat of the disc caused by the brake pads. The water is forced through the maze-like configuration where it is vaporized, utilizing the latent heat of vaporization of the water/steam to absorb the disc's heat.
In Kobelt, U.S. Pat. No. 4,013,148, a disc with zigzag cooling passages is disclosed as a means for keeping the brake system cool and thus retarding the wear of the brake pads. Neither Dunlop et al. or Kobelt illustrate or suggest in a liquid cooled disc the radially extending cooling cells and flow tubes to effectuate a continuous flow of cooling water while utilizing the latent heat of vaporization for maximal cooling of the present invention.
FIG. 8 shows a prior art device in Hikari, U.S. Pat. No. 4,217,775. As seen in FIG. 8 a disc with first and second internal compartments 12a, 14a, is separated by a partition 16a which extends radially through the disc 10a for cooling water to pass through. The water is fed into a supply duct 18a in the center of the disc 10a, where, by centrifugal force, and supply pressure the water is forced to the perimeter of the disc via the first compartment 12a where it is heated. The direction of water flow is indicated by the arrows shown. It is then discharged from the disc via the second compartment 14a and discharge duct 20a. Hikari, while showing the use of radially displaced cooling water internal to the disc for removing heat therefrom, does not teach or suggest the use of flow tubes and cooling cells to disperse cooling water internal to the disc as the present invention does.
Because a system such as a dynamometer is required to absorb large amounts of energy for an extended period of time, it is imperative that such a system employ an effective energy absorbing means. In a liquid-cooled system, the maximum heat transfer for a given change in temperature occurs at vaporization, a condition which is called latent heat of vaporization. Thus, internally liquid-cooled systems which operate at vaporization are generally most effective.
Although maximum heat transfer occurs at vaporization, other conditions result which can cause problems for internally liquid-cooled systems. More specifically, the liquid vaporization, if not allowed to expand generates a large amount of pressure which can block the entry of incoming liquid. If the cooling liquid is blocked, the vaporizated liquid will superheat thereby disabling the power absorbing means.
Furthermore, the vapor/liquid mix must exit the disc in such a way as to allow the disc's outer surfaces to remain dry. If the outer surfaces do not remain dry, the coefficient of friction between the outer surfaces and the braking pads will vary, resulting in non-uniform resistive force. Such non-uniform force is undesirable in a dynamometer.
Besides providing a power absorption means, a dynamometer must be able to determine the MEPD's output torque at a given angular velocity. Angular velocity can be measured in many straight forward ways. Output torque, on the other hand, is somewhat more complicated to measure accurately.
Akkerman et al., U.S. Pat. No. 3,940,978 discloses a water dynamometer which, through a measurement of the water pressure required as an opposing force, indicates the corresponding horsepower.
Hikari. U.S. Pat. No. 4,217,775 shows a load testing apparatus utilizing liquid-cooled disc brakes as a power absorbing means, as described above. Hikari employs a detector which is compressed by pressers to measure the output torque of the test engine. Inherent inaccuracies may exist in these measurement techniques.
The present invention is designed to overcome the limitations that are attendant to traditional dynamometers, and toward this end, it contemplates the provision of a dynamometer which can accurately test an MEPD with a high horsepower rating for an extended period of time.
Neither Akkerman nor Hikari teach or suggest the present invention's strategic placement of a sensing device along a resultant force which is a direct measure of the opposing torque.